There are a number of known methods and apparatus for continuously casting metal into strips, sheets and slabs. The term "metal" as used herein, refers to any type of castable metal, including, but not limited to, aluminum, steel, iron, copper, zinc, nickel, titanium, magnesium, manganese and their alloys. In a typical continuous casting process, molten metal is supplied from a tundish to a system of rollers, belts or chains which define a continuously moving mold. Block casters are particularly useful in continuously casting metal because they can provide a wide range of solidification rates, which allows a wide range of control over the physical properties of the metal being cast.
A typical block caster includes two synchronized, counter-rotating chains containing chilling blocks which travel through casting loops. The casting loops are disposed in close relation to one another such that the counter-rotating chains can be forced together to define a flat plane, continuously moving mold assembly for receiving molten metal. As the molten metal is poured from a tundish and contacts the surfaces of the mold, heat transfer between the molten metal and the mold surfaces causes the molten metal to solidify.
The counter-rotating beam chains in a block caster travel in a track which defines the shape of the casting loops. Typically, the casting loops are oval in shape, containing two substantially linear sections and two non-linear bends, however, other shapes have been employed. Generally, in one linear section of the casting loop, the chilling blocks are cooled and in the other linear section the chilling blocks define a casting region. The chain can be driven around the track through the use of a drive system, which typically is a system of gears or sprockets in mesh with the chain.
In known block casters, the chain is comprised of a number of chilling blocks, which are affixed to support beams. The chilling blocks define the continuously moving mold and are in direct contact with the molten metal. The support beams are typically used to interlink the chilling blocks together to form an endless "beam" chain and can contain features for meshing with the track and drive systems. The chilling blocks themselves are typically not interlinked or in mesh with the track and drive systems because the chilling blocks experience thermal and physical deformations during casting which could adversely affect the operation of the caster. Thus, it is desirable that the chilling blocks be at least partially thermally isolated from the support beams. For example, U.S. Pat. No. 3,570,586 by Lauener, assigned to Lauener Engineering Ltd., generally describes a block caster with chilling blocks thermally isolated from support beams, which travel through a casting loop along a guideway.
It is desirable in a continuous block caster to provide a substantially smooth, planar mold surface for casting metal sheets, strips or slabs. The amount by which of the mold surface approximates a smooth plane can have a direct impact on the surface quality and the microstructure of the cast. For example, changes in block height or block surface angle can create surface imperfections in the cast or can create insulating gas pockets between the block surface and the molten metal affecting the solidification rate of the metal and thus the microstructure of the cast.
U.S. Pat. No. 5,133,401 by Cisko et al., assigned to the Aluminum Company of America, discloses a block casting apparatus purportedly for solving the problem of poor surface accuracy of a cast slab. The disclosed apparatus utilizes a chilling block and support beam structure. The support beams contain inboard and outboard or "offset" rollers for carrying the beam chain along horizontal upper and lower guide tracks. The support beams are interconnected using elastic hinges to form an endless beam chain. The beam chain is driven around the guide tracks using an opposed-torque gearing system in mesh with gear racks which are located on the bottom surfaces of the support beams.
Known casting systems, however, such as that disclosed in by Cisko et al., allow individual chilling blocks to tilt around an axis (the "y-axis") transverse to the casting direction, negatively impacting the amount the mold surface approximates a smooth plane. The meshing of the gear rack system disclosed by Cisko et al. can be dependent upon manufacturing tolerances. Moreover, the offset roller system requires precise manufacturing tolerances of the rollers and the guide track to prevent binding or excess movement of the rollers in the track.
It is also generally desirable that a block caster contain features which accommodate the differences in track length and beam chain length. Differences in beam chain length and track length can occur when fitting beams in a chain and also during casting as a result of thermal effects upon the beam chain or the track. If these differences are not compensated-for, the blocks can move relative to one another in the casting region, reducing the quality of the cast through "banging," i.e., unnecessary contact between adjacent blocks, or by allowing molten metal to seep between chilling blocks causing damage to the caster and the chilling blocks. Damage to the caster and the chilling blocks causes lost production due to down-time required to repair the caster and/or to replace damaged chilling blocks.
In known block casters, such as that described in the '401 patent by Cisko, et al., elastic hinges have been used for interlinking the support beams to accommodate differences in beam chain and guide track lengths. The use of elastic hinges in the beam chain and an opposed-torque gear drive system, however, can cause problems in meshing the gear drive system with the gear racks on the support beams. Elastic hinge systems are designed to allow adjacent blocks to exert pressure upon one another in the casting region to prevent gaps between chilling blocks from forming. The use of an elastic beam chain alone, however does not compensate for reductions in the quality of the cast due to banging between blocks.
It is further desirable that a block caster be designed to substantially reduce imperfections in the cast and damage caused to chilling blocks caused by mechanical forces such as vibrations and the like propagated by blocks traveling along a track. Moreover, it is desirable to substantially reduce any additional forces or effects created by blocks traveling through a casting cycle which can negatively impact the quality of the cast.
The '401 patent by Cisko et al., previously described herein, also discloses the use of tracks which are asymmetrical about a plane parallel to a lateral plane through the mold cavity. Cisko et al. disclose that each bend in their elongated oval track consists of two smoothly joined quadrants each having a different radius and center, and that typically no two of the four radii of the four quadrants are the same.
The asymmetrical track design disclosed in the '401 patent by Cisko et al. purportedly minimized the "mechanical noise" generated by the "mechanical excitation" of the chilling blocks banging against each other in the bends of the track as can occur when using an elastic beam chain. The asymmetrical tracks are an attempt to reduce the net effects of mechanical excitation in the bends by maintaining the inputs from positive and negative block acceleration out of phase. The asymmetrical track design for dampening mechanical excitation described by Cisko et al., however, does not substantially compensate for other forces or effects which can negatively impact the quality of the cast which are propagated by chilling blocks traveling through a casting cycle.